This is misleading. There's no place in the specification of Haskell that specifically defines monads as part of the language. They aren't a language feature. They are a pattern which happens to be expressible in Haskell, and which is supported by libraries:
"Haskell's built in support for monads is split among the standard prelude, which exports the most common monad functions, and the Monad module, which contains less-commonly used monad functions. The individual monad types are each in their own libraries and are the subject of Part II of this tutorial."
The fact that monads are used to implement parts of the language doesn't make them a core part of it. Techniques used in implementation aren't the same as language features.
"Haskell's built in support for monads is split among the standard prelude, which exports the most common monad functions, and the Monad module, which contains less-commonly used monad functions. The individual monad types are each in their own libraries and are the subject of Part II of this tutorial."
https://wiki.haskell.org/All_About_Monads#Monad_support_in_H...
The fact that monads are used to implement parts of the language doesn't make them a core part of it. Techniques used in implementation aren't the same as language features.